In some seismic prospection methods, a seismic source is disposed in a borehole and the source is excited at given depths in order to produce well-determined acoustic waves in the adjacent underground formations, and a set of sensors is used, either spread over the surface of the ground or else disposed in other adjacent boreholes, in order to pick up acoustic information for analysis purposes after it has propagated through and been reflected from geological formations.
In the prior art, when a seismic source is deployed in a borehole, no special precautions have been taken with respect to the source concerning the way in which it is coupled to the geological formations situated at the same depth.
In particular, regardless of the type of source used, the acoustic wave(s) developed by exciting the source act both sideways towards said geological formations and upwardly and downwardly along the borehole in the form of compression waves and/or tube waves. This upwardly and downwardly propoagating acoustic energy sets up interference noise phenomena in the adjacent geological formations, and this noise is picked up by the sound sensors for providing the signals to be analyzed.
Further, for economic reasons, it is desirable to optimize the quantity of acoustic energy which is applied to the geological formations concerned. Thus, since a generally very large fraction of the acoustic energy propagates vertically along the borehole without substantial effect on the geological formation, it is necessary in practice, to repeat sound emission a large numer of times, and this is disadvantageous.
The object of the present invention is to mitigate these drawbacks and to provide a structure for confining the acoustic energy produced by the source, when excited, against the walls of the borehole at the depth of said source.